i3test: add kill_all_windows convenience function
i3test: add kill_all_windows convenience function

Airblader/i3-original,i3/i3,Airblader/i3-original,Airblader/i3,Airblader/i3-original,stapelberg/i3,i3/i3,stapelberg/i3,Airblader/i3,i3/i3,stapelberg/i3,i3/i3,stapelberg/i3,Airblader/i3-original,Airblader/i3,Airblader/i3,Airblader/i3-original